For dying the hair, a hair dye which utilizes an oxidative coupling reaction of an oxidation dye intermediate (i.e., a precursor and a coupler) under the coexistence of an alkaline agent and an oxidizing agent is widely used.
For the alkaline agent, ammonia is often utilized; however, the odor of ammonia becomes a problem. Although a number of solutions have been proposed for reducing the odor of ammonia, the reduction of the odor without impairing bleaching effect thereof is difficult.
It should also be noted that a hair dye containing an oxidizing agent often causes damage to the hair. As a consequence, not only lift-up or flake-away of the cuticle, production of cysteic acid and a reduction in the lipid inside the hair will result but a feeling and appearance of the dyed hair tend to get easily impaired. The impairment to the feeling and appearance is exemplified by a feeling that fingers do not run through the hair smoothly; a feeling of friction; and a rough feeling and so on when the hair is shampooed or dried, and also by deterioration of a hair color/sheen and manageability. Further, such damage to the hair and impairment to the feeling and appearance accumulates as hair dying and bleaching are repeated, which eventually creates an inequality in the degree of damage between the root and the tip of the hair. This causes problems such that unevenness is more likely to be resulted in hair dying, or colors on the hair tip are more easily to fade away by shampooing, etc.
As one method for resolving hair damage and impairment to the feeling and appearance, use of an additive having a conditioning effect has been practiced. For example, addition of a silicone derivative such as a certain kind of amino-modified polysiloxane to an oxidation hair dye (JP-A-S63-51315), and addition of a cationic polymer (U.S. Pat. No. 4,362,528) have been proposed. However, conditioning components as described above are still unsatisfactory due to a drawback that they end up reducing dyeing and bleaching effects when incorporated in an amount necessary to bring about a sufficient conditioning effect.
Alternatively, another method is proposed, in which a “pre-treatment agent” containing trimethylglycine is used before dying the hair to thereby improve the dying effect, which further suppresses hair damage whereby a conditioning effect is provided (JP-A-2005-255534). However, the application procedure disclosed in the Example (i.e., subsequent to application of a pre-treatment agent to the hair, a hair dye is applied without washing) may possibly reduce the dying effect because the concentration of the dye which actually works on the hair is diluted due to the pre-treatment agent applied. Furthermore, a drawback is accompanied such that the operation becomes complicated since application requires a two-step process.